The present invention relates to a continuously variable transmission for a vehicle of a type having a rotation transmission mechanism of variable ratio having at least reverse and forward low speed range of large reduction ratios and a forward high speed range of small reduction ratio and which is equipped with a planetary gear mechanism, a belt-type continuously variable transmission mechanism including an input pulley and an output pulley each having a stationary flange and a movable flange made axially movable with respect to the stationary flange, and rotation transmitting members of an endless belt type running on the two pulleys.
The aforementioned belt-type continuously variable transmission mechanism (referred to below as a "CVT") may be combined with a fluid coupling and a centrifugal clutch or the like thereby to construct a vehicular transmission having a gear ratio varied continuously and over a wide range. A speed ratio e=N'/N of the rotational speed N' of the output shaft of the CVT to the rotational speed N of the input shaft is large during high-speed running of the vehicle but is small for low-speed operation such as when the vehicle is just starting to move. As a result, when the vehicle abruptly comes to a halt after high-speed running, the rotation transmitting member (belt), which is constructed by connecting in the form of an endless belt blocks made to run on the input and output pulleys of the CVT, has a low speed. This makes it impossible to run the belt sufficiently for ensuring the speed ratio e necessary for restarting. As a result, the output torque at restart is so low that the vehicle has poor starting characteristics and poor initial acceleration and experiences acceleration shocks.
To ameliorate these disadvantages, there has been described in Japanese Patent Laid-Open Patent Application No. 56-66557 a system in which a torque converter is interposed between the engine output shaft and the CVT to amplify the engine torque thereby to improve the starting characteristics and initial acceleration. In the aforementioned system, however, a torque several times as high as the engine torque is applied to the CVT when the vehicle first starts to move. This can damage the blocks of the aforementioned belt and the input and output pulleys tend to have a short service life. In order to transmit the aforementioned high torque, moreover, the face pressure between the blocks and the pulleys is raised, making it necessary to provide an oil pump of high capacity.
With a view of improving the operability of the CVT, there has been described in Japanese Patent Laid-Open Patent Application No. 56-49452 an apparatus in which a CVT and a planetary gear mechanism are combined to widen the speed change ratio of the automatic transmission to a width derived by multiplying the speed change ratios of the CVT and the planetary gear mechanism. In this apparatus too there arises a problem similar to the aforementioned in that a torque exceeding the engine torque is applied to the CVT.